Abstract
The High Resolution Doppler Imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) has been measuring winds in the stratosphere, mesosphere and lower thermosphere since November, 1991. The winds are determined by measuring the Doppler shift of emission and absorption lines in the O<sub>2</sub> Atmospheric Band that are located between 630 and 762 nm. HRDI is a triple-etalon Fabry-Perot interferometer that has a resolution of ~0.05 cm<sup>-1</sup> and very good white light rejection. A multi-channel detector with 31 channels is used to examine a spectral region 0.5 cm<sup>-1</sup> wide and an adjustable filter wheel permits the selection of any one of 13 spectral bands. The long life of this instrument has presented many challenges in keeping the calibrations current and in compensating for inevitable degradations in instrument and spacecraft performance. Some of the problems with the UARS spacecraft the affect HRDI operations are: limited power due to the solar array drive failure; loss of data resulting from a failure of the tape recorders, and loss of attitude knowledge caused by the failure of the star trackers. HRDI has shown little loss in capability over the years with only a decrease in the azimuth rate of the telescope motor a significant sign of aging. This paper will discuss some of these challenges and how they have been met.
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